Modern phased array antennas are limited in their application primarily by cost. Even utilizing the latest MMIC technology, the required phase shifters have a unit cost in excess of $500. With a typical array requiring 3000 individual antenna elements, each with its own phase shifter, the array price quickly becomes prohibitive.
Numerous attempts have been made to lower the cost of phased array elements. Investigations were made into the use of PIN diodes, since the diodes lent themselves to an inexpensive phase shifter design. However, no way was discovered to avoid the high insertion losses associated with the diodes, especially at the Ku frequency band and above.
Ferrite phase shifters gained popularity in recent years, as initial problems of weight, size and operational speed were overcome. But unit cost and complexity have hindered them from becoming a preferred building block.
More recently, use of ferroelectric materials has been of interest. This is because certain dielectric properties of such materials change under the influence of an electric field. In particular, an electrooptic effect can be produced by the application of a bias electric field to ferroelectric materials. By electrooptically varying the refractive indices of such material, a phase shift will occur in electromagnetic radiation passing therethrough. The overall procedure is known as electrooptic phase-shifting.
Regions of ferroelectric materials have a non-zero electric dipole moment in the absence of an applied electric field. For this reason, ferroelectric materials are regarded as spontaneously polarized.
A suitably oriented polarized ferroelectric medium changes the propagation conditions of passing electromagnetic radiation. A bias electric field of sufficient magnitude in the appropriate direction may change the refractive index of the medium, thereby further altering the propagation conditions.
Upon incidence with a uniaxial ferroelectric medium having a suitably aligned optic axis, radiation divides into two components (i.e., double refraction). A first component exhibits polarization of the electric field perpendicular to the optic axis, and refracts in the medium according to Snell's Law (the ordinary ray). A second component exhibits polarization orthogonal to that of the first, with some constituent of the electric field parallel to the optic axis (the extraordinary ray). The extraordinary ray is refracted in a different manner, and may not behave according to Snell's Law.
The refractive indices of the ferroelectric material for the two wave components, n.sub.o and n.sub.e respectively, determine the different velocities of propagation of the components' phase fronts. The applied bias electric field typically changes the refractive indices, which causes phase shifts in the propagating radiation.
Examples of radar scanning devices which purported to take advantage of the foregoing principles of ferroelectric materials are disclosed and claimed in U.S. Pat. Nos. 4,636,799 and 4,706,094, both to Kubick, both assigned to the assignee of the present invention, and both of which are hereby incorporated by reference. Each patent describes and illustrates a monolithic piece of ferroelectric material disposed in front of a source of electromagnetic radio frequency ("RF") radiation. The material has a row of electrically conductive wires disposed on each side of the material and spanning the material from top to bottom. A DC voltage applied to the wires in a pattern produces a voltage gradient across the antenna aperture from one end to the other. Such a voltage gradient purportedly causes a gradient in the refractive index of the material, with a resulting shift in the radiation direction, thereby effectuating ferroelectric scanning.
Further, the ferroelectric material in Kubick U.S. Pat. No. 4,706,094 (the "electrooptic scanner patent") has an initial domain orientation parallel to the direction of propagation ("c-poled"), such c-poling being perpendicular to the surface of the ferroelectric material. With such c-poling, the radiation is affected only by the ordinary index of refraction, n.sub.o. However, it has been found experimentally that the electrooptic effect manifests itself more commonly in the extraordinary wave refractive index, n.sub.e. Thus, to achieve wave phase shifting, the polarization must be parallel to the optic axis, and, thus, to the bias electric field.
In co-pending U.S. patent application Ser. No. 07/791,842, to Collier et al., and assigned to the assignee of the present invention, a phased array antenna comprising an arrangement of ferroelectric material is disclosed and claimed. The antenna purports to take advantage of the experimental discovery above in which the polarization must be parallel to the optic axis, and, thus, to the bias electric field in order to achieve phase shifting of RF energy. An obstacle to the use of ferroelectric material in such phased array antennas has been the lack of apparatus for accurately measuring the phase-shifting ability (electrooptic activity) of such material at RF frequencies.